Progress of hydration reactions in olivine-H2O and orthopyroxenite-H2O systems at 250 °C and vapor-saturated pressure
Progress of hydration reactions in olivine-H2O and orthopyroxenite-H2O systems at 250 °C and vapor-saturated pressure
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DOI：
10.1016/j.chemgeo.2011.08.007
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发表时间：
2011-10-23
期刊：
影响因子：
3.9
通讯作者：
Tsuchiya, Noriyoshi
中科院分区：
文献类型：
--
作者：
Okamoto, Atsushi;Ogasawara, Yuichi;Tsuchiya, Noriyoshi
我们在250 ℃和蒸汽饱和压力（P-sat）条件下，在橄榄石（01; Fo（91））-H2O和斜方辉石（Opx;由95%的斜方辉石组成，En（66））-H2O系统中进行了水热实验，以检查溶液化学和产物的时间演化，持续时间长达1008 h。最大的水化程度（即。固体样品中的H2O含量）（3.6重量%）比Opx-H2O实验中的（0.4重量%）高得多。在Ol-H2O实验中，溶液中的Mg和Si显示出在降低（阶段II）之前的初始增加（阶段I），并且最终在504 h后达到稳定状态（阶段111）。随着二氧化硅活性向水镁石稳定场水平下降，产品也从蛇纹石+磁铁矿（第一和第二阶段）变为蛇纹石+水镁石+磁铁矿（第三阶段）。蛇纹石矿物也从利蛇纹石（阶段和II）到利蛇纹石+温石棉（阶段III）。在1008小时（水镁石+蛇纹石在橄榄石接触，和蛇纹石在最外缘）的产品周围橄榄石的分区模式是一致的产品的矿物学的时间变化，是类似的模式中观察到的网格边缘部分蛇纹状纯橄榄岩。在Opx-H2O实验中，斜方辉石颗粒形成后，滑石和蛇纹石的形成后，斜方辉石（辉长岩），如在天然水合方辉橄榄岩中观察到的不同。Opx-H_2 O体系的硅活性是Ol-H_2 O体系的10-10（3）倍，表明当Ol-H_2 O体系与Opx-H_2 O体系相连时，橄榄岩水化过程中水镁石不会在橄榄石之后形成。在这项研究中观察到的水化反应在反应物中产生了分级断裂，其中充满了反应产物，类似于天然橄榄岩中橄榄石之后的网状纹理。这种反应诱导的压裂产生了新的反应表面和流体通道，增强了水化反应。在Ol-H2O实验中生成蛇纹石+水镁石的整个反应显示出大的体积膨胀（在阶段III，V/V-0 = 1.43），而仅生成蛇纹石的反应以接近恒定的体积进行（在阶段I，V/V-0 = 1.19）。在大洋岩石圈中，蛇纹岩化过程中的体积膨胀比我们的实验结果要困难得多。因此，在反应位点处防止体积膨胀的情况下，发生以下结果之一：（1）水合反应停止，直到形成新的裂缝，或（2）通过从体系中除去Mg，反应以低体积膨胀（不存在水镁石）进行。这两种结果将在大洋岩石圈中产生截然不同的水化分布或程度。(C)2011爱思唯尔有限公司版权所有。
We conducted hydrothermal experiments in olivine (01; Fo(91))-H2O and orthopyroxenite (Opx; composed of 95% of orthopyroxene, En(66))-H2O systems under conditions of 250 degrees C and vapor-saturated pressure (P-sat) to examine the temporal evolution of the solution chemistry and products in runs of up to 1008 h in duration. The maximal degree of hydration (i.e.. H2O content in the solid sample) in the Ol-H2O experiments (3.6 wt.%) was much higher than that in the Opx-H2O experiments (0.4 wt.%). In the Ol-H2O experiments, Mg and Si in solution showed an initial increase (stage I) before decreasing (stage II) and finally attaining a steady state after 504 h (stage 111). Following a drop in silica activity toward the level of brucite stability field, the products also changed from serpentine + magnetite (stages I and II) to serpentine + brucite + magnetite (stage III). Serpentine minerals also changed from lizardite (stages land II) to lizardite + chrysotile (stage III). The zoning pattern of the products around olivine at 1008 h (brucite + serpentine at the olivine contact, and serpentine at the outermost rim) is consistent with the temporal changes in the mineralogy of the products, and is similar to the pattern observed in the mesh rims in partly serpentinized dunites. In the Opx-H2O experiments, chlorite formed after orthopyroxene grains, which differs from the formation of talc and serpentine after orthopyroxene (bastite), as observed in natural hydrated harzburgites. The Opx-H2O system maintained 10-10(3) times higher silica activity than Ol-H2O system, suggesting that brucite does not form after olivine during hydration of peridotites when the Ol-H2O system is linked to the Opx-H2O system.The progress of hydration reactions is affected by mechanical properties of host rocks. The hydration reactions observed in this study produced hierarchical fractures in the reactants, which became filled with reaction products, similar to mesh textures after olivine in natural peridotites. This reaction-induced fracturing produced new reaction surfaces and fluid pathways that enhanced the hydration reactions. The overall reaction producing serpentine + brucite in the Ol-H2O experiments showed the large volume expansion (V/V-0 = 1.43 at stage III), whereas that producing only serpentine proceeded with near constant volume (V/V-0 = 1.19 at stage I). The volume expansion is more difficult to occur in the oceanic lithosphere than in our experiments during serpentinization. Thus, in the case that volume expansion is prevented at reaction sites, one of the following outcomes occurs: (1) the hydration reaction stops until new fractures form, or (2) the reaction proceeds with low volume expansion (absence of brucite) by removing Mg from the system. These two outcomes would produce contrasting distributions or extent of hydration in oceanic lithosphere. (C) 2011 Elsevier B.V. All rights reserved.